Syndiotactic polystyrene (sPS) was first synthesized in 1986 (Ishihara et al., Macromolecules 1986, 19, 2464), using the homogeneous organometallic catalytic system based on a titanium compound and methylaluminoxane (MAO). Syndiotactic polystyrene is a very attractive polymer. The polymer shows a low specific gravity, a low dielectric constant, a high modulus of elasticity and an excellent resistance to chemicals. Accordingly the syndiotactic polystyrene has become a promising material for various applications in the automotive, electronic and packaging industries.
However, the commercialization of a syndiotactic polystyrene has some serious problems, i.e. serious reactor fouling or lump, and low flowability of the product powder resulted from its unsatisfactory morphology. The problems are remained unsolvable if only using a homogeneous catalyst. These problems are solved by using a catalyst supported to an organic or inorganic support instead of using a general homogeneous catalyst on polymerization. However, the activity of a supported catalyst is generally much lower than that of the corresponding homogeneous catalyst by the order of magnitude of 2–3, and the polymerization activity of a syndiotactic polystyrene is, generally, much lower than that of polyolefin. Therefore it is very difficult to prepare a supported catalyst having acceptable activity for producing a syndiotactic polystyrene.
So far, four basic methods have been developed for metallocene catalyst systems for production of polyolefin as follow:
1. direct adsorption of metallocene into the support surface involving physisorption or chemisorption of metallocene (direct heterogenization);
2. initial adsorption of methylaluminoxane (MAO) into the support, followed by adsorption of metallocene (indirect heterogenization);
3. covalent bonding of metallocene to a carrier by a ligand, followed by activation with MAO; and
4. use of an organic compound which is able to react with the hydroxyl group of an inorganic support surface such as silica and to form a complex with metallocene to be supported, which is represented by the following reaction as one example:Si—OH+HO—R—OH→Si—O—R—OH→Si—R—O . . . Metallocene
where R is a hydrocarbon compound.
Either the direct loading of a metallocene catalyst on a support (Method 1) or the indirect loading on a MAO treated support (Method 2, Kaminsky et al., J. Polym. Sci.: Part A: Polym. Chem. 1999, 37, 2959) does not provide a good activity for styrenic polymerization. Method 3 relates to a complex chemistry and difficulties arise when bonding the metallocene to the support surface. A spacer between support and metallocene was introduced in Method 4, but the results, as reported by Spitz et al. (Macromol. Chem. Phys. 1999, 200, 1453), show that there is no any enhancement of styrene polymerization activity.
Until now, very few reports can be seen in the area of supported catalyst for producing syndiotactic polystyrene. Silica (Kaminsky et al., J. Polym. Sci.: Part A: Polym. Chem. 1999, 37, 2959), alumina (Spitz et al., Macromol. Chem. Phys. 1999, 200, 1453) and polymer (Yu et al., J. Polym. Sci.: Part A: Polym. Chem. 1996, 34, 2237) have been used as a support for preparation of a supported catalyst for producing syndiotactic polystyrene. Unfortunately, all these supported catalysts are not applicable because of extremely low activity. Therefore, a supported catalyst with high activity for producing syndiotactic styrenic polymer is highly expected.
Accordingly, the present inventors have developed a supported metallocene catalyst with a high activity in combination with a cocatalyst for preparing a styrenic polymer with a high syndiotacticity.